The term "aramid" is used to designate wholly aromatic polyamides. Not all aramid fibers are useful in the present invention but only those derived from aromatic polyamides whose chain extending bonds are either coaxial or parallel and oppositely directed. High strength, high modulus aramid fibers useful in the present invention may be prepared by the processes described in U.S. Pat. Nos. 3,767,756 and 3,869,430. These fibers will be referred to hereinafter as p-aramid fibers. Particularly preferred are p-aramid fibers based on poly(p-phenylene terephthalamide) as produced by Du Pont under the trademark Kevlar.RTM..
Composite sheets of continuous p-aramid filament reinforced resin have been made. One technique is to prepare a warp of the filaments as by winding on a frame, impregnating with resin and hot-pressing to form a thin flat sheet which is cut from the frame. Several such sheets are then cross-lapped and again hot-pressed to form the final reinforced composite product. Such composites have high tensile strength and stiffness.
Problems occur when attempts are made to produce three-dimensional articles, particularly those with sharp bends, by hot-pressing continuous p-aramid filament-containing resin sheets. The shaped articles exhibit uneven areas and wrinkles as well as slack filaments. This is an indication of inadequate conformance to the mold.
The use of staple p-aramid fiber as reinforcement substantially overcomes the wrinkling problem referred to above but at a great sacrifice to strength and stiffness of the composites. Use of certain stretch broken, high strength, high modulus p-aramid fibers as the reinforcement has now been found to alleviate the aforementioned problems.